Loss of prion protein control of glucose metabolism promotes neurodegeneration in model of prion diseases

RNA-sequencing transcriptomic analysis of scrapie-exposed ovine mesenchymal stem cells

In neurodegenerative diseases, including prion diseases, cellular models arise as useful tools to study the pathogenic mechanisms occurring in these diseases and to assess the efficacy of potential therapeutic compounds. In the present study, a RNA-sequencing analysis of bone marrow-derived ovine mesenchymal stem cells (oBM-MSCs) exposed to scrapie brain homogenate was performed to try to unravel genes and pathways potentially involved in prion diseases and MSC response mechanisms to prions. The oBM-MSCs were cultured in three different conditions (inoculated with brain homogenate of scrapie-infected sheep, with brain homogenate of healthy sheep and in standard growth conditions without inoculum) that were analysed at two exposure times: 2 and 4 days post-inoculation (dpi). Differentially expressed genes (DEGs) in scrapie-treated oBM-MSCs were found in the two exposure times finding the higher number at 2 dpi, which coincided with the inoculum removal time. Pathways enriched in DEGs were related to biological functions involved in prion toxicity and MSC response to the inflammatory environment of scrapie brain homogenate. Moreover, RNA-sequencing analysis was validated amplifying by RT-qPCR a set of 11 DEGs with functions related with prion propagation and its associated toxicity. Seven of these genes displayed significant expression changes in scrapie-treated cells. These results contribute to the knowledge of the molecular mechanisms behind the early toxicity observed in these cells after prion exposure and to elucidate the response of MSCs to neuroinflammation.

Post-translational modifications in prion diseases

More than 650 reversible and irreversible post-translational modifications (PTMs) of proteins have been listed so far. Canonical PTMs of proteins consist of the covalent addition of functional or chemical groups on target backbone amino-acids or the cleavage of the protein itself, giving rise to modified proteins with specific properties in terms of stability, solubility, cell distribution, activity, or interactions with other biomolecules. PTMs of protein contribute to cell homeostatic processes, enabling basal cell functions, allowing the cell to respond and adapt to variations of its environment, and globally maintaining the constancy of the milieu interieur (the body's inner environment) to sustain human health. Abnormal protein PTMs are, however, associated with several disease states, such as cancers, metabolic disorders, or neurodegenerative diseases. Abnormal PTMs alter the functional properties of the protein or even cause a loss of protein function. One example of dramatic PTMs concerns the cellular prion protein (PrPC), a GPI-anchored signaling molecule at the plasma membrane, whose irreversible post-translational conformational conversion (PTCC) into pathogenic prions (PrPSc) provokes neurodegeneration. PrPC PTCC into PrPSc is an additional type of PTM that affects the tridimensional structure and physiological function of PrPC and generates a protein conformer with neurotoxic properties. PrPC PTCC into PrPSc in neurons is the first step of a deleterious sequence of events at the root of a group of neurodegenerative disorders affecting both humans (Creutzfeldt-Jakob diseases for the most representative diseases) and animals (scrapie in sheep, bovine spongiform encephalopathy in cow, and chronic wasting disease in elk and deer). There are currently no therapies to block PrPC PTCC into PrPSc and stop neurodegeneration in prion diseases. Here, we review known PrPC PTMs that influence PrPC conversion into PrPSc. We summarized how PrPC PTCC into PrPSc impacts the PrPC interactome at the plasma membrane and the downstream intracellular controlled protein effectors, whose abnormal activation or trafficking caused by altered PTMs promotes neurodegeneration. We discussed these effectors as candidate drug targets for prion diseases and possibly other neurodegenerative diseases.

Αnti-prion effects of anthocyanins

Prion diseases, also known as Transmissible Spongiform Encephalopathies (TSEs), are protein-based neurodegenerative disorders (NDs) affecting humans and animals. They are characterized by the conformational conversion of the normal cellular prion protein, PrPC, into the pathogenic isoform, PrPSc. Prion diseases are invariably fatal and despite ongoing research, no effective prophylactic or therapeutic avenues are currently available. Anthocyanins (ACNs) are unique flavonoid compounds and interest in their use as potential neuroprotective and/or therapeutic agents against NDs, has increased significantly in recent years. Therefore, we investigated the potential anti-oxidant and anti-prion effects of Oenin and Myrtillin, two of the most common anthocyanins, using the most accepted in the field overexpressing PrPScin vitro model and a cell free protein aggregation model. Our results, indicate both anthocyanins as strong anti-oxidant compounds, upregulating the expression of genes involved in the anti-oxidant response, and reducing the levels of Reactive Oxygen Species (ROS), produced due to pathogenic prion infection, through the activation of the Keap1-Nrf2 pathway. Importantly, they showcased remarkable anti-prion potential, as they not only caused the clearance of pathogenic PrPSc aggregates, but also completely inhibited the formation of PrPSc fibrils in the Cerebrospinal Fluid (CSF) of patients with Creutzfeldt-Jakob disease (CJD). Therefore, Oenin and Myrtillin possess pleiotropic effects, suggesting their potential use as promising preventive and/or therapeutic agents in prion diseases and possibly in the spectrum of neurodegenerative proteinopathies.

Interplay of mitochondria-associated membrane proteins and autophagy: Implications in neurodegeneration

Since the discovery of membrane contact sites between ER and mitochondria called mitochondria-associated membranes (MAMs), several pieces of evidence identified their role in the regulation of different cellular processes such as Ca2+ signalling, mitochondrial transport, and dynamics, ER stress, inflammation, glucose homeostasis, and autophagy. The integrity of these membranes was found to be essential for the maintenance of these cellular functions. Accumulating pieces of evidence suggest that MAMs serve as a platform for autophagosome formation. However, the alteration within MAMs structure is associated with the progression of neurodegenerative diseases. Dysregulated autophagy is a hallmark of neurodegeneration. Here, in this review, we highlight the present knowledge on MAMs, their structural composition, and their roles in different cellular functions. We also discuss the association of MAMs proteins with impaired autophagy and their involvement in the progression of neurodegenerative diseases such as Alzheimer's disease and Parkinson's disease.

A PrP EGFR signaling axis controls neural stem cell senescence through modulating cellular energy pathways

Mis-folding of the prion protein (PrP) is known to cause neurodegenerative disease; however, the native function of this protein remains poorly defined. PrP has been linked with many cellular functions, including cellular proliferation and senescence. It is also known to influence epidermal growth factor receptor (EGFR) signaling, a pathway that is itself linked with both cell growth and senescence. Adult neural stem cells (NSCs) persist at low levels in the brain throughout life and retain the ability to proliferate and differentiate into new neural lineage cells. KO of PrP has previously been shown to reduce NSC proliferative capacity. We used PrP KO and WT NSCs from adult mouse brain to examine the influence of PrP on cellular senescence, EGFR signaling, and the downstream cellular processes. PrP KO NSCs showed decreased cell proliferation and increased senescence in in vitro cultures. Expression of EGFR was decreased in PrP KO NSCs compared with WT NSCs and additional supplementation of EGF was sufficient to reduce senescence. RNA-seq analysis confirmed that significant changes were occurring at the mRNA level within the EGFR signaling pathway and these were associated with reduced expression of mitochondrial components and correspondingly reduced mitochondrial function. Metabolomic analysis of cellular energy pathways showed that blockages were occurring at critical sites for production of energy and biomass, including catabolism of pyruvate. We conclude that, in the absence of PrP, NSC growth pathways are downregulated as a consequence of insufficient energy and growth intermediates.

Promotive role of eukaryotic translation initiation factor 4A isoform 3 in ovarian cancer cell growth and aerobic glycolysis through the pyruvate dehydrogenase kinase 4 signaling

Ovarian cancer (OC) represents one of the most detrimental gynecological malignancies. RNA-binding protein eukaryotic translation initiation factor 4A isoform 3 (EIF4A3) is well-regarded as a definitive oncogene that contributes to the development of multiple malignant tumors. This study sought to elucidate the molecular mechanism of EIF4A3 in OC growth and aerobic glycolysis by regulation of pyruvate dehydrogenase kinase 4 (PDK4) mRNA stability. We determined the EIF4A3 and PDK4 expression levels in OC cell lines and normal ovarian epithelial cells, and subsequently evaluated the cell viability and colony formation by cell counting kit-8 and colony formation assays. The degree of cell aerobic glycolysis was evaluated by measurements of lactic acid production, glucose intake, adenosine triphosphate level, extracellular oxygen consumption, and protein levels of pyruvate kinase isozymes M2 and hexokinase-2. Afterwards, we verified the binding of EIF4A3 and PDK4 mRNA via RNA immunoprecipitation, and determined the mRNA stability after actinomycin D treatment. Finally, a series of rescue experiments was performed with pcDNA3.1-PDK4. EIF4A3 and PDK4 were upregulated in OC cells. Silencing EIF4A3 obstructed cell proliferation and aerobic glycolysis, while the same was annulled by EIF4A3 overexpression. Mechanically, EIF4A3 could bind to PDK4 mRNA to stabilize its mRNA and upregulate its protein levels. PDK4 overexpression inverted the inhibitory role of silencing EIF4A3 in proliferation and aerobic glycolysis. Overall, our findings highlighted that EIF4A3 induced OC progression by stabilizing PDK4 mRNA.

Inhibition of androgen receptor enhanced the anticancer effects of everolimus through targeting glucose transporter 12

Everolimus was designed as a mammalian target of rapamycin (mTOR) inhibitor. It has been proven as a targeted drug for gastric cancer (GC) therapy. However, long-term treatment with everolimus may cause severe side effects for recipients. Decreasing the dosage and attenuating the associated risks are feasible to promote clinical translation of everolimus. This study aimed to identify the underlying mechanisms of responses to everolimus and develop novel regimens for GC treatment. Our findings proved that there was a significant dose-dependent relationship of everolimus-induced GC cell apoptosis and glycolysis inhibition. Then, we found that a member of glucose transporter (GLUT12) family, GLUT12, was actively upregulated to counteract the anticancer effects of everolimus. GLUT12 might be overexpressed in GC. High expression of GLUT12 might be correlated with tumor progression and short survival time of GC patients. Bioinformatic analysis suggested that GLUT12 might be involved in regulating cancer development and metabolism. The experiments proved that GLUT12 significantly promoted GC growth, glycolysis and impaired the anticancer effects of everolimus. Androgen receptor (AR) is a classical oncogenic factor in many types of cancer. Everolimus elevated GLUT12 expression in an AR-dependent manner. Inhibition of AR activity abrogated the promotive effects on GLUT12 expression. Both in-vitro and in-vivo experiments demonstrated that GLUT12 knockdown augmented anticancer effects of everolimus. Enzalutamide, an AR inhibitor, or AR knockdown was comparable to GLUT12 suppression. This study identified the role of the AR/GLUT12 pathway in the development of poor responses to everolimus. Interference with AR/GLUT12 pathway may serve as a promising approach to promoting the translational application of everolimus in GC therapy.

Impairment of Neuronal Mitochondrial Quality Control in Prion-Induced Neurodegeneration

Mitochondrial dynamics continually maintain cell survival and bioenergetics through mitochondrial quality control processes (fission, fusion, and mitophagy). Aberrant mitochondrial quality control has been implicated in the pathogenic mechanism of various human diseases, including cancer, cardiac dysfunction, and neurological disorders, such as Alzheimer’s disease, Parkinson’s disease, and prion disease. However, the mitochondrial dysfunction-mediated neuropathological mechanisms in prion disease are still uncertain. Here, we used both in vitro and in vivo scrapie-infected models to investigate the involvement of mitochondrial quality control in prion pathogenesis. We found that scrapie infection led to the induction of mitochondrial reactive oxygen species (mtROS) and the loss of mitochondrial membrane potential (ΔΨm), resulting in enhanced phosphorylation of dynamin-related protein 1 (Drp1) at Ser616 and its subsequent translocation to the mitochondria, which was followed by excessive mitophagy. We also confirmed decreased expression levels of mitochondrial oxidative phosphorylation (OXPHOS) complexes and reduced ATP production by scrapie infection. In addition, scrapie-infection-induced aberrant mitochondrial fission and mitophagy led to increased apoptotic signaling, as evidenced by caspase 3 activation and poly (ADP-ribose) polymerase cleavage. These results suggest that scrapie infection induced mitochondrial dysfunction via impaired mitochondrial quality control processes followed by neuronal cell death, which may have an important role in the neuropathogenesis of prion diseases.

Melatonin: Regulation of Prion Protein Phase Separation in Cancer Multidrug Resistance

The unique ability to adapt and thrive in inhospitable, stressful tumor microenvironments (TME) also renders cancer cells resistant to traditional chemotherapeutic treatments and/or novel pharmaceuticals. Cancer cells exhibit extensive metabolic alterations involving hypoxia, accelerated glycolysis, oxidative stress, and increased extracellular ATP that may activate ancient, conserved prion adaptive response strategies that exacerbate multidrug resistance (MDR) by exploiting cellular stress to increase cancer metastatic potential and stemness, balance proliferation and differentiation, and amplify resistance to apoptosis. The regulation of prions in MDR is further complicated by important, putative physiological functions of ligand-binding and signal transduction. Melatonin is capable of both enhancing physiological functions and inhibiting oncogenic properties of prion proteins. Through regulation of phase separation of the prion N-terminal domain which targets and interacts with lipid rafts, melatonin may prevent conformational changes that can result in aggregation and/or conversion to pathological, infectious isoforms. As a cancer therapy adjuvant, melatonin could modulate TME oxidative stress levels and hypoxia, reverse pH gradient changes, reduce lipid peroxidation, and protect lipid raft compositions to suppress prion-mediated, non-Mendelian, heritable, but often reversible epigenetic adaptations that facilitate cancer heterogeneity, stemness, metastasis, and drug resistance. This review examines some of the mechanisms that may balance physiological and pathological effects of prions and prion-like proteins achieved through the synergistic use of melatonin to ameliorate MDR, which remains a challenge in cancer treatment.